I like liberalism, BUT...

The United Nations cannot be classified under any one group. With that said, to answer this question is difficult. But since I have to answer the question, my answer is that the United Nations is an organization that attempts to balance realism and liberalism. From here let me explain.
The United Nations was founded in 1945 after World War II by the Allied Nations in order to “maintain international peace and security, and to that end: to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace, and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace” (UN Charter, Chapter 1) This is a pretty realist idea. To have a group of nations work on maintaining world security. This is the first purpose of the United Nation as stated in the charter. But the charter continues with “To develop friendly relations among nations based on respect for the principle of equal rights and self-determination of peoples, and to take other appropriate measures to strengthen universal peace;” (UN Charter, Chapter 1) This is a very liberal idea. The UN wants to help create “friendly relations” based off equality in the people. Realists don’t really care about the people. Liberals do.
The UN tries to protect the entire world, but that is difficult with conflicting world interests. To be a realist, the UN would have to overlook the self interests of the nations involved and just look over security. For the UN to be a liberal, it would have to not put security as its number one. The UN does however make it a top priority since it is the first purpose of the UN. But the UN also helps more in humanitarian efforts. To define the UN as either one or the other is very hard since the UN can’t even make up its mind on what issues it wants to tackle and how.